The compound 3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-6-butyric acid (DBA) has been shown by others to possess anti-sickling activity with intact SS erythrocytes. The intracellular polymerization of deoxy-Hb S which occurs upon deoxygenation is responsible for the sickling phenomenon. The microtubular fibers so produced result from the profoundly diminished solubility of deoxy-Hb S relative to oxy-Hb S. Accordingly, the utility of DBA as a noncovalent inhibitor of the polymerization of deoxy-Hb S was evaluated by in vitro studies. The effect of DBA, alone and in combination with ethanol, on the solubility of deoxy-Hb S was studied by two types of solubility assay: (1) measurement of the saturation concentration, csat, under conditions approaching physiological and (2) salting out by exposure to high ionic strength phosphate buffer. Determinations of csat required concentrated solutions of Hb S (32-36 g/dl) which were dialyzed against Bis-Tris buffer, pH 6.75 and equilibrated for 2 h at 30 prior to phase separation by centrifugation at 30 degrees. Plots of csat versus DBA concentration, in the presence or absence of ethanol, gave two parallel lines, indicative of the additive fashion in which ethanol and DBA increase the solubility of deoxy-Hb S, presumably by destabilizing noncovalent hydrophobic interactions in the polymeric fiber. In the absence of ethanol, at a DBA/Hb molar ratio of 10:1, csat was increased 16 percent. Ethanol alone increased csat comparably, but at a much higher molar excess (200:1). Complementary studies using the salting out method showed that a 10-fold molar excess of DBA had no effect on the solubility of deoxy-Hb S. Hence, no correlation exists between the effect of DBA on the solubility of deoxy-Hb S measured as csat and that measured by salting out. Oxygen dissociation curves were measured under standard conditions. In the presence of a 10-fold molar excess of DBA, the oxygenation parameters were virtually identical to the control (P 50 equals 10 mm; n equals 2.9). Hence, DBA does not influence the solubility of Hb S by altering its oxygen affinity. Rather, it appears to interfere directly with the mechanism of polymerization.